This invention relates to novel soft contact lenses and to the method of and apparatus for making same. Lenses having aspheric curvature are disclosed in the Evans Pat. No. 4,199,231, the disclosure of which is incorporated herein by reference, and it is to be noted that this invention represents an improvement over such patent.
The aspheric curves according to the Evans patent were developed particularly to provide visual correction for presbyopes, with or without correction for astigmatism, or for prepresbyopic astigmats. Correctly fitted, these lenses provide excellent correction from near or reading distance to infinity for such users.
The aspheric curves according to the Evans patent are expressed in polar coordinate form in which the radius of curvature, p, is of the form: EQU p=R+kR[1-cos (a)]/[1+cos (a)] where:
k is an empirically derived value lying between 0.005 and 0.1 and is determined from the characteristics of the lens material and (a) is the polar angle.
These aspheric curvatures have proven to be particularly difficult to generate and virtually impossible to cut reproducibly. In consequence, these lenses have not met with wide acceptance even though when correctly generated and fitted, they provide excellent optical performance which cannot be duplicated by any other lens currently available. The reason for the difficulty in generating these lenses is that the asphericity, particularly at values of (a) up to about 10.degree. (20.degree. optic zone) are of very small amounts. The Evans patent aspheric surface is of ellipsoidal configuration deviating from a spherical surface only by amounts which are measured accurately only in microns or fractions thereof. For example, for an aspheric lens of this type having a radius at the center of the lens of 8.0 mm, the maximum asphericity or deviation from this radius at the peripheral edge of the lens [angle(a)=35.degree.] is equal to:
k(0.795306610148), which upon substituting the normally used value of k (0.015) yields 0.0119 mm as the maximum asphericity E'. For small values of (a), the asphericities will be so small as are barely measurable much less capable of accurate generation. For example, at a value of (a) of 5.degree., the asphericity in the above case will be 0.0002 mm. Even though contact Lens lathes, such as the Hardinge lathe, are available which will very accurately generate or cut a spherical surface, no technique has been found which will generate the Evans aspheric surfaces accurately and reproducibly.